1. Field of the Invention
The present invention relates in general to pre-charge control systems of an automatic transmission, which, upon issuance of gear change instruction, quicken starting of stroke action of a friction element which is to be engaged, and more particularly to the pre-charge control systems of a type which controls the stroke action of the friction element when various (for example, two) types of gear change instructions, each needing an engaged condition of a same friction element, are issued in succession.
2. Description of the Related Art
As is known, for establishing a desired gear speed in an automatic transmission, friction elements such as clutches and brakes are selectively engaged with the aid of hydraulic power. More specifically, upon issuance of a gear change instruction for a desired gear speed, a switching is carried out from a friction element (or elements) which is to be released to a friction element (or elements) which is to be engaged, for establishing the desired gear speed.
In general, in order to determine a gear speed suitable for the existing driving condition of an associated motor vehicle, a shift map, such as one shown in FIG. 7 is employed. That is, based on such shift map, a controller (viz., computer) looks up a suitable gear speed with reference to drive condition representing factors, such as an engine throttle opening TVO and a vehicle speed VSP, and in accordance with the suitable gear speed thus looked up, the controller carries out a switching from a selected friction element which is to be released to a selected friction element which is to be engaged, for establishing the looked up gear speed.
In the following, description will be made with respect to a transitional condition of a transmission wherein for carrying out a desired gear change, one friction element is being released by lowering the hydraulic pressure applied thereto and the other friction element is being engaged by increasing the hydraulic pressure applied thereto. An instruction value of hydraulic pressure for one friction element (viz., releasing-side friction element) and that of hydraulic pressure for the other friction element (viz., engaging-side friction element) is denoted by xe2x80x9cPoxe2x80x9d and xe2x80x9cPcxe2x80x9d respectively, which are depicted by a time chart of FIG. 8. For ease of understanding of the following description, the values xe2x80x9cPoxe2x80x9d and xe2x80x9cPcxe2x80x9d will be referred to as releasing-side instruction value and engaging-side instruction value respectively.
First, the engaging-side instruction value xe2x80x9cPcxe2x80x9d will be described with reference to the time chart of FIG. 8. As shown in the time chart, for a given period xe2x80x9cC1xe2x80x9d from time xe2x80x9ct1xe2x80x9d at which a gear change instruction is issued, the engaging-side instruction value xe2x80x9cPcxe2x80x9d is instantly raised like a step and kept high (viz., pre-charge pressure) for the purpose of quickening starting of stroke action of the engaging-side friction element. That is, the given period xe2x80x9cC1xe2x80x9d is regarded as a given pre-charge period.
Then, for a period xe2x80x9cC2xe2x80x9d subsequent to the given pre-charge period xe2x80x9cC1xe2x80x9d, the engaging-side instruction value xe2x80x9cPcxe2x80x9d is instantly lowered from the pre-charge pressure and gradually increased with a rate of change that does not bring about undesired engaging shock of the engaging-side friction element. The pressure exerted in the period xe2x80x9cC2xe2x80x9d is thus referred to as a stand-by pressure. It is to be noted that if the stroke action of the friction element is fully carried out by the pre-charge pressure, a marked shift shock is generated upon engagement of the friction element.
At a time xe2x80x9ct2xe2x80x9d, that is, at a terminal end of the period xe2x80x9cC2xe2x80x9d, the stroke action of the engaging-side friction element is finished thereby starting to have an engaging capacity, and thus, the engaging side-friction element starts to enter a torque phase. That is, from time xe2x80x9ct2xe2x80x9d, the engaging-side instruction value xe2x80x9cPcxe2x80x9d is increased with an illustrated time series change rate thereby to increase the engaging capacity of the engaging-side friction element.
Next, the releasing-side instruction value xe2x80x9cPoxe2x80x9d will be described. As shown in the time chart of FIG. 8, for a given period xe2x80x9cO1xe2x80x9d from time xe2x80x9ct1xe2x80x9d at which the gear change instruction is issued, the releasing-side instruction value xe2x80x9cPoxe2x80x9d is instantly lowered like a step and kept at a given level for the purpose of ensuring a releasing response of the releasing-side friction element. Then, for periods xe2x80x9cO2xe2x80x9d and xe2x80x9cO3xe2x80x9d subsequent to the period xe2x80x9cO1xe2x80x9d, the releasing-side instruction value xe2x80x9cPoxe2x80x9d is gradually lowered with a smaller rate of change thereby to control or lower the actual hydraulic pressure in such a manner as to gradually induce a slip operation of the releasing-side friction element.
Then, from time xe2x80x9ct2xe2x80x9d at which, due to completion of the stroke action, the engaging-side friction element starts to have the engaging capacity, the releasing-side instruction value xe2x80x9cPoxe2x80x9d is further lowered with an illustrated time series change rate thereby to gradually lower the engaging capacity of the releasing-side friction element.
As is understood from the above description, upon switching operation from the releasing-side friction element to the engaging-side friction element, the torque phase starts at time xe2x80x9ct2xe2x80x9d and advances until completion of the switching operation thereby to accomplish a desired gear change.
Now, consideration will be directed to a gear change that would take place when, as is indicated by an arrow in the shift map of FIG. 7, an accelerator pedal is depressed for increasing the throttle opening TVO. That is, at a time xe2x80x9ct1xe2x80x9dof the time chart of FIG. 9 when a shift point line N-1 of the shift map of FIG. 7 is crossed by time xe2x80x9ct1xe2x80x9d, an instruction gear speed becomes (N-1)-speed and thus a gear change instruction for changing from existing N-speed to (N-1)-speed is issued, and then at a time xe2x80x9ct2xe2x80x9d of the time chart of FIG. 9 when another shift point line N-2 of the shift map of FIG. 7 is crossed by time xe2x80x9ct2xe2x80x9d, an instruction gear speed becomes (N-2)-speed and thus a gear change instruction for changing from the existing N-speed to (N-2)-speed is issued. That is, upon depression of the accelerator pedal in the above-mentioned manner, two gear change instructions, namely, xe2x80x9cNxe2x86x92(N-1)xe2x80x9d gear change instruction and xe2x80x9cNxe2x86x92(N-2)xe2x80x9d gear change instruction, are inevitably issued in succession.
However, when the xe2x80x9cNxe2x86x92(N-1)xe2x80x9d gear change instruction and the xe2x80x9cNxe2x86x92(N-2)xe2x80x9d gear change instruction, which are issued in succession, are of a type that needs an engaged condition of a same friction element, a known control system carries out an operation wherein the above-mentioned pre-charge control is forced to start at both times xe2x80x9ct1xe2x80x9d and xe2x80x9ct2xe2x80x9d of the time chart of FIG. 9. In this case, however, if the pre-charge control starting at time xe2x80x9ct1xe2x80x9d when the xe2x80x9cNxe2x86x92(N-1)xe2x80x9d gear change instruction is issued is not finished at time xe2x80x9ct2xe2x80x9d when the xe2x80x9cNxe2x86x92(N-2)xe2x80x9d gear change instruction is issued, the period for which the pre-charge control is carried out becomes excessively long and thus the stroke action of the friction element becomes too rapid due to the excessive pre-charging. In this case, engaging shock of the friction element tends to occur, and due to too early engagement of the engaging-side friction element as compared with the releasing of the releasing-side friction element, a gear change shock tends to occur due to an interlock tendency. If, for elimination of this drawback, the pre-charge control is forced to finish at time xe2x80x9ct2xe2x80x9d when the xe2x80x9cNxe2x86x92(N-2)xe2x80x9d gear change instruction would be issued, the stroke action of the engaging-side friction element becomes slow due to lack of the pre-charging period, which tends to induce undesired engine roaring.
It is therefore an object of the present invention to provide a pre-charge control system of an automatic transmission, which is free of the above-mentioned drawbacks.
According to a first aspect of the present invention, there is provided a pre-charge control system for use in an automatic transmission. The transmission includes a plurality of friction elements which are selectively engageable to establish various gear speeds, a gear change system which, upon issuance of a gear change instruction, applies hydraulic pressure to selected one of the friction elements to induce an engaged condition of the same thereby to establish a certain gear speed and a pre-charge arrangement which, for quickening starting of engaging stroke of the friction element in the gear change operation, raises an instruction value of hydraulic pressure for the selected friction element to a pre-charge pressure value upon issuance of the gear change instruction and keeps the instruction value at the pre-charge pressure value for a pre-charge period. The pre-charge control system controls operation of the pre-charge system and comprises a first section which receives first and second gear change instructions which are issued in succession, the first and second gear change instructions being of a type of needing an engaged condition of the friction element; and a second section which, when the second gear change instruction is issued during a pre-charge period of the friction element which is needed by the first gear change instruction, extends the pre-charge period of the selected friction element to a time when a pre-charge period needed by the second gear change instruction lapses.
According to a second aspect of the present invention, there is provided a pre-charge control system for use in an automatic transmission. The transmission includes a friction element of a double piston type having two pistons, a gear change system which, upon issuance of a gear change instruction, induces an engaged condition of the friction element to establish a certain gear speed of the transmission and a pre-charge system which, for quickening starting of engaging stroke of the friction element, raises an instruction value of hydraulic pressure for the friction element to a pre-charge pressure value upon issuance of the gear change instruction and keeps the instruction value at the pre-charge pressure value for a pre-charge period. The pre-charge control system controls operation of the pre-charge system and comprises a first section which receives first and second gear change instructions which are issued in succession, the first and second gear change instructions being of a type of needing the engaged condition of the friction element; and a second section which, when the second gear change instruction is issued during a pre-charge period of the friction element which is needed by the first gear change instruction, extends the pre-charge period of the friction element to a time when a pre-charge period needed by the second gear change instruction lapses.